Sorting of a Single-Walled Carbon Nanotubes Using Optical Dipole Traps

ABSTRACT

In embodiments of the present invention, the electric field of a focused laser beam induces a dipole in a single-walled carbon nanotube. The single-walled carbon nanotube has one or more resonant frequencies. When the frequency of the laser beam is less than a resonance frequency of the single-walled carbon nanotube, the single-walled carbon nanotube may be trapped and the laser beam may move the single-walled carbon nanotube from a first microfluidic laminar flow to a second microfluidic laminar flow. When the frequency of the laser beam is higher than a resonant frequency of the single-walled carbon nanotube, the single-walled carbon nanotube may be repelled and the laser beam may not move the single-walled carbon nanotube.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of and claims benefitof priority to U.S. application Ser. No. 10/107,833, filed Mar. 26,2002.

BACKGROUND

1. Field

Embodiments of the present invention relate to single-walled carbonnanotubes and, in particular, to sorting single-walled carbon nanotubes.

2. Discussion of Related Art

Carbon nanotubes have evoked considerable interest since their discoveryin the early 1990s. Potential uses include everything from transistors,digital memory, and miniature electron emitters for displays, tohydrogen gas storage devices for the next generation of environmentallyfriendly automobiles.

Typically, a batch of single-walled carbon nanotubes available topotential users has a mixture of different types of single-walled carbonnanotubes. For example, in a batch of single-walled carbon nanotubesthere may be metallic single-walled carbon nanotubes and semiconductorsingle-walled carbon nanotubes. Within the semiconductor single-walledcarbon nanotubes there may be single-walled carbon nanotubes ofdifferent lengths, diameters, and/or chiralities. Each type ofsingle-walled carbon nanotube has different properties (e.g.,electrical, chemical, optical, mechanical) that are particularlysuitable for different applications. Because they usually come as amixture not being able to separate the different single-walled carbonnanotubes can be troublesome when attempting to utilize a particulartype of single-walled carbon nanotube for a specific application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally equivalent elements. Thedrawing in which an element first appears is indicated by the leftmostdigit(s) in the reference number, in which:

FIG. 1 is a flow chart illustrating process for sorting semiconductorsingle-walled carbon nanotubes from metallic single-walled carbonnanotubes according to an embodiment of the present invention;

FIG. 2 is a two-dimensional diagram of a hexagonal lattice of asingle-walled carbon nanotube showing the chiral vector and the chiralangle;

FIG. 3 is a graphical representation showing dielectric susceptibilitywith respect to resonant frequency;

FIG. 4 is a graphical representation showing electron density of statesfor a semiconductor single-walled carbon nanotube;

FIG. 5 is a graphical representation showing bandgap electron density ofstates for a metallic single-walled carbon nanotube;

FIG. 6 is a graphical representation showing the relationship betweenbandgap and diameter for single-walled carbon nanotubes;

FIG. 7 is a top view of a microfluidic system according to an embodimentof the present invention;

FIG. 8 is a flow chart illustrating process for sorting semiconductorsingle-walled carbon nanotubes according to an embodiment of the presentinvention;

FIG. 9 is a top view of a microfluidic system suitable for implementingthe process illustrated in FIG. 8 according to an embodiment of thepresent invention;

FIG. 10 is a top view of a microfluidic system suitable for sortingsemiconductor single-walled carbon nanotubes according to an alternativeembodiment of the present invention; and

FIG. 11 is a high-level block diagram of a system suitable for usingsingle-walled carbon nanotubes sorted according to embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 is a flow chart illustrating a process 100 for sortingsemiconductor single-walled carbon nanotubes from metallic single-walledcarbon nanotubes according to an embodiment of the present invention.The process 100 will be described as multiple discrete operationsperformed in turn in a manner that is most helpful in understandingembodiments of the present invention. However, the order in which theoperations are described should not be construed to imply that theoperations are necessarily order dependent or that they be performed inthe order in which they are presented. Of course, the process 100 isonly an example process and other processes may be used.

In a block 102, the process 100 determines diameter and chirality (e.g.,arm chair, zigzag, helical (or chiral)) corresponding a target class ofsingle-walled carbon nanotubes. A single-walled carbon nanotube can bemodeled as a strip of graphite sheet rolled up into a seamless cylinderwith or without end caps. It is “single-walled” because its wall is onlya single atom thick. The cylinder is generated when a graphene sheet iswrapped such that an atom on one edge of the sheet coincides with anatom on the other edge of the sheet. The vector pointing from the firstatom towards the second atom is called the chiral vector and the lengthof the “chiral vector” is equal to the circumference of thesingle-walled carbon nanotube. The direction of the single-walled carbonnanotube axis is perpendicular to the chiral vector.

FIG. 2 is a two-dimensional diagram 200 of a hexagonal lattice of asingle-walled carbon nanotube showing the chiral vector and the chiralangle. The chiral vector C_(h) is defined on the hexagonal lattice asC_(h)=nâ₁+mâ₂, where â₁ and â₂ are unit vectors, and n and m areintegers. The chiral angle θ is measured relative to the directiondefined by â₁. The example diagram 200 has been constructed for (n,m)=(4, 2), and the unit cell of this single-walled carbon nanotube isbounded by OAB'B. To form the single-walled carbon nanotube, imaginethat this cell is rolled up so that O meets A and B meets B′, and thetwo ends are capped.

Different types of carbon nanotubes have different values of n and m.Zigzag nanotubes correspond to (n, O) or (O, m) and have a chiral angleof 0°, armchair nanotubes have (n, n) and a chiral angle of 30°, whilechiral nanotubes have general (n, m) values and a chiral angle ofbetween 0° and 30°. In embodiments of the present invention, scanningtunneling microscopy may be used to determine and display atomicstructures for single-walled carbon nanotubes.

Single-walled carbon nanotubes that have different lengths, diameters,and/or chiral vectors have different electronic properties. For example,depending on their chiral vector, single-walled carbon nanotubes withsmall diameters are either semiconductor single-walled carbon nanotubesor metallic single-walled carbon nanotubes. Metallic single-walledcarbon nanotubes may conduct electricity at room temperature.Semiconductor single-walled carbon nanotubes do not conduct at roomtemperature.

Referring back to FIG. 1, in a block 104, the process 100 identifies aresonant frequency of the target class of single-walled carbonnanotubes. In one embodiment, a laser beam emitting a particularfrequency of light may be scanned across the mixture of single-walledcarbon nanotubes. The electric field component of the light interactswith one or more single-walled carbon nanotubes. The electric fieldcomponent induces dipole moments in the target single-walled carbonnanotubes.

When the frequency of the laser beam is equivalent to a resonantfrequency of the target single-walled carbon nanotubes, the induceddipoles in the target single-walled carbon nanotubes will resonate. Whenthe frequency of the laser beam is lower than a resonant frequency ofthe single-walled carbon nanotubes, the single-walled carbon nanotubeswill be attracted to the laser beam (i.e., optically trapped). When thefrequency of the laser beam is higher than a resonant frequency of thesingle-walled carbon nanotubes, the laser beam will repel thesingle-walled carbon nanotubes. (When the frequency of the laser beam isaround a resonant frequency of the single-walled carbon nanotubes, theoptical trapping of single-walled carbon nanotubes is unstable.)

The induced dipole moment of the neutral particle in the electric fieldmay be represented by P=ε₀χE. P is the dipole moment per unit volume (orpolarization) of the neutral particle in the electric field of the laserbeam. ε₀ is the permittivity of free space and is a constant. χ(ω) isthe dielectric susceptibility of a neutral particle to become a dipole(i.e., to become polarized). The dielectric susceptibility χ(ω) dependson the electronic structure of the particle as well as the surroundingmedium (e.g., free space, water, etc).

FIG. 3 is a graphical representation 300 showing dielectricsusceptibility χ of a neutral particle (i.e., a single-walled carbonnanotube) to become polarized with respect to laser beam frequency ω.Dielectric susceptibility is a complex function (χ(ω)=χ′(ω))+iχ″(ω),that depends on the frequency ω of the laser beam attempting to polarizeit. The graphical representation 300 shows an imaginary part 302 to thedielectric susceptibility, which is always positive, and a real part304, which changes sign when crossing a resonant frequency (point 306).

The sign of the real part of dielectric susceptibility χ′(ω) determineswhether the single-walled carbon nanotube will be attracted to orrepelled from the electric field component of the laser beam. If thesign of the real part χ′(ω) is positive, then the laser beam willattract the single-walled carbon nanotube. If the sign of the real partof χ′(ω) is negative, then laser beam will repel the single-walledcarbon nanotube. The sign of the real part of dielectric susceptibilityχ′(ω) is determined by the frequency ω of the laser beam. If thefrequency ω of the laser beam is lower than the resonant frequency ofthe single-walled carbon nanotube, then the sign of the real part ofdielectric susceptibility χ′(ω) is positive. If the frequency ω of thelaser beam is higher than the resonant frequency of the single-walledcarbon nanotube, then the sign of the real part of dielectricsusceptibility χ′(ω) is negative.

The total interaction system energy U (i.e., the energy that is aproduct of the dipole moment and the electric field of the laser beam)may be represented by U=−½P·E, where E is the electric field componentof the laser beam. The total interaction system energy U also may berepresented by U=−½ε₀χE². The total interaction system energy U thusdepends on the sign of the real part of dielectric susceptibility χ′(ω)and the intensity of the electric field component of the laser beam E².For a positive χ′(ω), U will decrease with increasing laser intensityE². Thus, a nanotube with positive χ′(ω) will tend to move to an area ofhigher laser intensity. For a focused laser beam, the intensitydistribution is normally Gaussian with highest intensity point at thecenter of the laser beam, where a neutral particle (i.e. nanotube) witha positive χ′(ω) will be most stable (i.e. lowest system energy U). Thisis the principle of optically induced dipole (optical dipole) traps.

Referring back to FIG. 1, in a block 106, the process 100 determines therelationship between the resonant frequency and the diameter andchirality of the target class of single-walled carbon nanotubes. FIG. 4is a graphical representation showing band structure 400 (energy/γ₀ withrespect to density of states (DOS) of single-walled carbon nanotube andgraphite; the dotted line represents graphite) for an examplesemiconductor single-walled carbon nanotube.

For example, the semiconductor single-walled carbon nanotube bandgapstructure 400 includes several peak pairs that are notated as E₁₁ ^(S),E₂₂ ^(S), etc. These peak pairs are van Hove singularities in theone-dimensional electronic density of states (DOS) for a single-walledcarbon nanotube. E₁₁ ^(S) is the first energy separation (or bandgap).E₂₂ ^(S) is the second energy separation (or bandgap). Each peak pairrepresents a resonant frequency for the single-walled carbon nanotube.Thus, E₁₁ ^(S) represents the first bandgap and the first resonantfrequency for the semiconductor single-walled carbon nanotube and E₂₂^(S) represents the second bandgap and the second resonant frequency forthe semiconductor single-walled carbon nanotube.

FIG. 5 is a graphical representation showing band structure 500(energy/γ₀ with respect to density of states (DOS) of single-walledcarbon nanotube and graphite) for a metallic single-walled carbonnanotube. The metallic single-walled carbon nanotube bandgap structure500 also includes several peak pairs that are notated as E₁₁ ^(M), E₂₂^(M), etc. E₁₁ ^(M) is the first energy separation and E₂₂ ^(M) is thesecond energy separation. Each peak pair represents a resonant frequencyfor the single-walled carbon nanotube. Thus, E₁₁ ^(M) represents thefirst bandgap and the first resonant frequency for the metallicsingle-walled carbon nanotube. Note that there are two peak pairsnotated as E₁₁ ^(M), which may be due to splitting of van Hovesingularities caused by trigonal warping effect (i.e., asymmetry nearFermi point in one dimensional electronic structure of the single-walledcarbon nanotube).

Note also that the metallic peak pair E₁₁ ^(M) is much larger than thesemiconductor peak pair E₁₁ ^(S) and is even larger than thesemiconductor E₂₂. In embodiments of the present invention, scanningtunneling microscopy may be used to determine and display atomicstructures and electronic density of states for single-walled carbonnanotubes.

The energy gaps between the corresponding van Hove singularities areoptically allowed inter-band transition energies. The inter-bandtransition energies are determined by the diameter and chirality of eachsingle-walled carbon nanotube. FIG. 6 is a graphical representationillustrating a plot 600 of single-walled carbon nanotube resonantfrequencies (i.e., inter-band transition energies) with respect tosingle-walled carbon nanotube diameter.

The plot 600 shows the calculated energy separations E_(ii) (e.g., E₁₁,E₂₂, E₃₃, etc.) between van Hove singularities in the one-dimensionalelectronic density of states (DOS) of the conduction and valence bandsfor all (n, m) values of considered single-walled carbon nanotube havingdiameters d_(t) in the range of approximately 0.4<d_(t)<3.0 nanometers(nm). The value for the single-walled carbon-carbon energy overlapintegral γ₀ is 2.9 eV. The nearest neighbor carbon-carbon distancea_(c-c) is 1.42 angstroms (Å). The index i in the inter-band transitionsE_(ii) denotes the transition between the ith van Hove singularities,with i=1 being closest to the Fermi energy level taken at E =0.

If Eii is known, then the diameter and chirality of a particularsingle-walled carbon nanotube can be determined. Note that the bandgapof the single-walled carbon nanotube is inversely proportional to itsdiameter. For example, in metallic single-walled carbon nanotubes E₁₁^(M)≅6 γ₀ a_(c-c)/d_(t). In semiconductor nanotubes E₁₁ ^(S)≅2 γ₀a_(c-c)/d_(t). This means that as the diameter of the single-walledcarbon nanotube increases, the bandgap and resonant frequency decreases.

Depending on chirality, the inter-band transition energies Eii of thesingle-walled carbon nanotube may deviate from being inverselyproportional to the diameter (i.e., may deviate from E₁₁ ^(M)≅6 γ₀a_(c-c)/d_(t) and E₁₁ ^(S)≅2 γ₀ a_(c-c)/d_(t)). As described above, thisis due to splitting (in metallic single-walled carbon nanotubes) orshifting (in semiconductor single-walled carbon nanotubes) of van Hovesingularities due to trigonal warping effect. This trigonal warpingeffect happens only if n is not equal m (i.e., armchair single-walledcarbon nanotubes).

Referring back to FIG. 1, in a block 108, a mixture of single-walledcarbon nanotubes is disposed in a layer in a microfluidic system. Themixture of single-walled carbon nanotubes includes at least one targetsingle-walled carbon nanotube.

FIG. 7 is a top view of a microfluidic system 700 according to anembodiment of the present invention. The mixture of single-walled carbonnanotubes in the microfluidic system 700 includes several single-walledcarbon nanotubes 702, 704, 706, 708, 710, 712, 714, and 716. In theillustrated embodiment, the single-walled carbon nanotubes 714 and 716are target single-walled carbon nanotubes.

The microfluidic system 700 may include one or more layers (two layers720 and 722 are shown for simplicity, flowing in a direction 732 and734, respectively) of viscous fluid (e.g., water) flowing smoothlyadjacent to each other in laminar flow. In one embodiment, the mixtureof single-walled carbon nanotubes may be disposed in the layer 720.

In one embodiment, the width and height of any one of the layers 720 and722 may be less than approximately one millimeter (mm).

The flow of the fluid in any one of the layers 720 or 722 can becharacterized by the Reynolds number RN, which is represented byRN=ρνd/θ, where ρ is the fluid density, ν is the fluid speed, η is theviscosity, and d is a geometrical dimensions associated with the flow(e.g., the width and height of the layer). When the Reynolds number isbelow approximately 2000, the fluid flow is laminar. When the Reynoldsnumber is above approximately 2000, the fluid flow is turbulent.

The example microfluidic system 700 also includes a laser beam 730.Referring back to FIG. 1, in a block 110, the laser beam 730 is directedat the mixture of single-walled carbon nanotubes. The laser beam 730 mayhave a frequency that is lower than a resonant frequency of the targetsingle-walled carbon nanotubes. The laser beam 730 induces electricdipoles in the target single-walled carbon nanotubes and traps thetarget single-walled carbon nanotubes.

In one embodiment, the single-walled carbon nanotubes 714 and 716 arethe target semiconductor single-walled carbon nanotubes and the laserbeam 730 traps the semiconductor single-walled carbon nanotubes 714 and716.

Single-walled carbon nanotubes produced using arc discharge, laserablation, chemical vapor deposition (CVD), or other methods have acertain distribution of diameters. For example, the distribution ofsingle-walled carbon nanotubes made by known high pressure COdisproportionation processes (HiPCO) have diameters that range fromapproximately 0.8 to 1.3 nm (see area 602). In one embodiment, theenergy of the laser beam is chosen below 0.55 eV. In this embodiment,most of the metallic and semiconductor single-walled carbon nanotubesmay be trapped because the laser frequency is below the resonantfrequency for single-walled carbon nanotubes for E₁₁ ^(M) and E₁₁ ^(S)(i.e., all of the single-walled carbon nanotubes may be trapped).

In an alternative embodiment, the energy of the laser beam is chosen ataround 1.05 eV. In this embodiment, most of the semiconductorsingle-walled carbon nanotubes are released and all metallicsingle-walled carbon nanotubes may be trapped because the laserfrequency is below the resonant frequency for single-walled carbonnanotubes for E₁₁ ^(M) and above the resonant frequency forsingle-walled carbon nanotubes for E₁₁ ^(S) (i.e., traps may be createdin only the metallic single-walled carbon nanotubes).

In another embodiment, after the metallic single-walled carbon nanotubesare sorted from the semiconductor single-walled carbon nanotubes theenergy in the laser beam is tuned between 0.6-1 eV. In this embodiment,some semiconductor single-walled carbon nanotubes are released and somesemiconductor single-walled carbon nanotubes are trapped because thelaser frequency is below the resonant frequency for some single-walledcarbon nanotubes for E₁₁ ^(S) and above the resonant frequency for othersingle-walled carbon nanotubes for E₁₁ ^(S) (i.e., some semiconductorsingle-walled carbon nanotubes may be trapped but other semiconductorsingle-walled carbon nanotubes may not be trapped).

In a block 112, the semiconductor single-walled carbon nanotubes 714 and716 are moved from the layer 720 to the layer 722 and the metallicsingle-walled carbon nanotubes 702, 704, 706, 708, 710, 712 remain inthe layer 720. For example, the focal point of the laser beam 730 maymove from the layer 720 to the layer 722. Recall from above that asingle-walled carbon nanotube with positive χ′(ω) will tend to move toan area of higher laser intensity, which, for a focused laser beam whoseintensity distribution is Gaussian, is at the center of the laser beam.Thus, the movement of the focal point of the laser beam 730 from thelayer 720 to the layer 722 may cause the semiconductor single-walledcarbon nanotubes 714 and 716 to move from the layer 720 to the layer722.

In one embodiment, a mirror (not shown) may be used to change theposition of the focal point of the laser beam 730. For example, theangle of the mirror may be continuously or incrementally changed tochange the angle of deflection of the laser beam 730.

In a block 114, the process 100 collects the semiconductor single-walledcarbon nanotubes 714 and 716 from the layer 722 in one collection place736 and the metallic single-walled carbon nanotubes 702, 704, 706, 708,710, 712 from the layer 720 in another collection place 738.

For purposes of illustrating an alternative embodiment in which it isappropriate to separate group B semiconductor single-walled carbonnanotubes from a mixture of groups A, B, and C semiconductorsingle-walled carbon nanotubes. Suppose that a resonant frequency of Asemiconductor single-walled carbon nanotubes is higher than a resonantfrequency of B semiconductor single-walled carbon nanotubes and aresonant frequency of B semiconductor single-walled carbon nanotubes ishigher than a resonant frequency of C semiconductor single-walled carbonnanotubes.

FIG. 8 is a flow chart illustrating process 800 for sorting Bsemiconductor single-walled carbon nanotubes from the A and Csemiconductor single-walled carbon nanotubes according to an embodimentof the present invention. FIG. 9 is a top view of a microfluidic system900 suitable for implementing the process 800 according to an embodimentof the present invention.

The process 800 will be described as multiple discrete operationsperformed in turn in a manner that is most helpful in understandingembodiments of the present invention. However, the order in which theoperations are described should not be construed to imply that theoperations are necessarily order dependent or that they be performed inthe order in which they are presented. Of course, the process 800 isonly an example process and other processes may be used.

In a block 802, the laser beam 930 is tuned to a frequency ω₁ that islower than the resonant frequency of the B semiconductor single-walledcarbon nanotubes and higher than C semiconductor single-walled carbonnanotubes.

In a block 804, the mixture of A, B, and C semiconductor single-walledcarbon nanotubes is disposed in a first microfluidic layer 920 inlaminar flow.

In a block 806, the mixture of A, B, and C semiconductor single-walledcarbon nanotubes flows to the laser beam 930 and the laser beam 930traps the A and B single-walled carbon nanotubes.

In a block 808, the focal point of the laser beam 930 moves, which movesthe A and B single-walled carbon nanotubes to a second microfluidiclayer 922 in laminar flow while the C single-walled carbon nanotubesremain in the first microfluidic layer 920.

In a block 810, the laser beam 930 is tuned to a frequency ω₂ that ishigher than the resonant frequency of the B semiconductor single-walledcarbon nanotubes but that is lower than the resonant frequency of the Asemiconductor single-walled carbon nanotubes.

In a block 812, the mixture of the A and B semiconductor single-walledcarbon nanotubes flows to the laser beam 930 and the laser beam 930traps the A single-walled carbon nanotubes.

In a block 814, the laser beam 930 moves the A semiconductorsingle-walled carbon nanotubes to a third microfluidic layer 924 inlaminar flow while the B single-walled carbon nanotubes remain in thesecond microfluidic layer 922.

In a block 816, the process 800 collects the A semiconductorsingle-walled carbon nanotubes from the third layer 924 in a collector926 and collects the B semiconductor single-walled carbon nanotubes fromthe second layer 922 in a collector 932.

FIG. 10 illustrates a microfluidic system 1000 suitable for sorting A,B, and C semiconductor single-walled carbon nanotubes according to analternative embodiment of the present invention. This embodiment may beappropriate when the resonant frequency of the A semiconductorsingle-walled carbon nanotubes is higher than the resonant frequency ofthe B semiconductor single-walled carbon nanotubes and the resonantfrequency of the B semiconductor single-walled carbon nanotubes ishigher than the resonant frequency of the C semiconductor single-walledcarbon.

The laser beam 1002 may be tuned to a frequency ω₁ that is lower thanthe resonant frequency of the A semiconductor single-walled carbonnanotubes but higher than the resonant frequency of the B semiconductorsingle-walled carbon nanotubes. The laser beam 1002 may trap the Asemiconductor single-walled carbon nanotubes and move the Asemiconductor single-walled carbon nanotubes from the microfluidic layer1004 to the microfluidic layer 1006 to the collector 1008. The B and Csemiconductor single-walled carbon nanotubes remain in the microfluidiclayer 1004.

The laser beam 1010 may then be tuned to a frequency ω₂ that is lowerthan the resonant frequency of the B semiconductor single-walled carbonnanotubes but higher than the resonant frequency of the C semiconductorsingle-walled carbon nanotubes. The laser beam 1010 may trap the Bsemiconductor single-walled carbon nanotubes and move B semiconductorsingle-walled carbon nanotubes from the microfluidic layer 1004 to themicrofluidic layer 1006 to the collector 1012. The C semiconductorsingle-walled carbon nanotubes may remain in the microfluidic layer1004.

The laser beam 1014 may be tuned to a frequency ω₃ that is lower thanthe resonant frequency of the C semiconductor single-walled carbonnanotubes. The laser beam 1014 may trap the C semiconductorsingle-walled carbon nanotubes and move the C semiconductorsingle-walled carbon nanotubes from the microfluidic layer 1004 to themicrofluidic layer 1006 to the collector 1016.

It is very common for the batch of single-walled carbon nanotubesprovided by single-walled carbon nanotube manufacturers to potentialusers to aggregate together and form bundles similar to ropes due tovery strong van der Waals forces. In one embodiment of the presentinvention, a batch of single-walled carbon nanotubes may befunctionalized prior to sorting using optical dipole traps. For example,the batch of single-walled carbon nanotubes is dispersed in an aqueoussurfactant solution to un-bundle them. Suitable surfactants are known(e.g., sodium dodecyl sulfate (SDS)).

In one embodiment of the present invention, a tunable laser provides thelaser beam. The tunable laser therefore may be able to scan acrossfrequencies or switch among frequencies. In an alternative embodiment,multiple lasers may be used to provide the laser beams. The laser beamsmay have the same frequency to provide high efficiency trapping of atarget class of single-walled carbon nanotubes (i.e., the laser beamsmay be directed toward the target class of carbon nanotubessimultaneously so that substantially all of the target class ofsingle-walled carbon nanotubes may be trapped in one pass).Alternatively still, the multiple laser beams having the same frequencymay be directed toward the target class of carbon nanotubes sequentiallyso that substantially all of the target class of single-walled carbonnanotubes may be trapped in a serial manner. The laser beam sweepingspeed across the microfluidic flows may be fast enough so that all ofthe target single-walled carbon nanotubes may be removed in one pass.

Although for simplicity only one microfluidic system is described, inembodiments, several microfluidic systems may be used to sortsingle-walled carbon nanotubes. For example, two or more microfluidicsystems can be implemented in parallel with each other. Alternatively,two or more microfluidic systems can be implemented in series with eachother. After reading the description herein, a person of ordinary skillin the relevant art will readily recognize how to implement embodimentsof the present invention using two or more microfluidic systems.

FIG. 11 is a high-level block diagram of a system 1100 suitable forusing single-walled carbon nanotubes sorted according to embodiments ofthe present invention. The system 1100 may be a scanning tunnelingmicroscope including a single-walled carbon nanotube tip 1102, apiezoelectric tube 1104 coupled to the control the distance of the tip1102 from a sample 1106, which is in dotted lines because it is not partof the system 1100, a tunneling current amplifier 1108 to amplifytunneling current from the tip 1102, a control unit 1110 coupled toprovide voltage to electrodes 1112 and 1114 on the tube 1104, and adisplay 1116 to display results of scanning the sample 1106. Scanningtunneling microscopes suitable for using single-walled carbon nanotubessorted according to embodiments of the present invention are known.

Other systems suitable for using single-walled carbon nanotubes sortedby bandgap (i.e., optical dipole resonant frequency) according toembodiments of the present invention include transistor fabricationsystems. For example, in many devices transistor bandgap is controlledso that all transistors on a particular device have the same bandgap.This ensures that all transistors have the same threshold voltage. Othersystems suitable for using single-walled carbon nanotubes sortedaccording to embodiments of the present invention include batterymanufacturing systems, and fuel cell manufacturing systems.

Embodiments of the present invention may be implemented using hardware,software, or a combination thereof. In implementations using software,the software may be stored on a machine-accessible medium. Amachine-accessible medium includes any mechanism that provides (i.e.,stores and/or transmits) information in a form accessible by a machine(e.g., a computer, network device, personal digital assistant,manufacturing tool, any device with a set of one or more processors,etc.). For example, a machine-accessible medium includes recordable andnon-recordable media (e.g., read only memory [ROM], random access memory[RAM], magnetic disk storage media, optical storage media, flash memorydevices, etc.), as well as electrical, optical, acoustic, or other formof propagated signals (e.g., carrier waves, infrared signals, digitalsignals, etc.).

The above description of illustrated embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes, variousequivalent modifications are possible within the scope of theembodiments of the invention, as those skilled in the relevant art willrecognize. These modifications can be made to embodiments of theinvention in light of the above detailed description.

In the above description, numerous specific details, such as particularprocesses, materials, devices, and so forth, are presented to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the embodiments of thepresent invention can be practiced without one or more of the specificdetails, or with other methods, components, etc. In other instances,well-known structures or operations are not shown or described in detailto avoid obscuring the understanding of this description.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, process, block,or characteristic described in connection with an embodiment is includedin at least one embodiment of the present invention. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification does not necessarily meanthat the phrases all refer to the same embodiment. The particularfeatures, structures, or characteristics may be combined in any suitablemanner in one or more embodiments.

The terms used in the following claims should not be construed to limitembodiments of the present invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope ofembodiments of the invention is to be determined entirely by thefollowing claims, which are to be construed in accordance withestablished doctrines of claim interpretation.

1. A method, comprising: directing a laser beam at a mixture of carbonnanotubes disposed in a microfluidic layer in laminar flow, the laserbeam having a frequency less than a resonant frequency of at least onetarget class of carbon nanotubes, the resonant frequency determined bydiameter and chirality of the target class of carbon nanotubes, themixture including at least one target class of carbon nanotube; trappingat least one target carbon nanotube; and moving the target carbonnanotube into a second microfluidic layer in laminar flow.
 2. The methodof claim 1, further comprising identifying the resonant frequency of thetarget class of carbon nanotubes.
 3. The method of claim 2, furthercomprising determining diameter and chirality of the target class ofcarbon nanotubes.
 4. The method of claim 3, further comprisingdetermining a relationship between diameter, chirality, and resonantfrequency of the target class of carbon nanotubes.
 5. The method ofclaim 1, further comprising collecting at least one target carbonnanotube from the second microfluidic layer in laminar flow.
 6. Themethod of claim 1, further comprising un-bundling the mixture of carbonnanotubes.
 7. The method of claim 1, wherein the target class of carbonnanotubes are metallic single-walled carbon nanotubes.
 8. The method ofclaim 1, wherein the target class of carbon nanotubes are semiconductorsingle-walled carbon nanotubes.
 9. The method of claim 1, wherein, themixture of carbon nanotubes includes another target class of carbonnanotubes, the method further comprising: directing a laser beam havinganother laser frequency at the mixture of carbon nanotubes, the anotherlaser frequency being less than a next resonant frequency of the nexttarget class of carbon nanotubes, the other resonant frequencydetermined by diameter and chirality of the other target class of carbonnanotubes; trapping the other target class of carbon nanotubes; andmoving the other target class of carbon nanotubes into a thirdmicrofluidic layer.
 10. The method of claim 1, further comprising:directing another laser beam having a laser frequency less than theresonant frequency of the target class of carbon nanotubes at themixture of carbon nanotubes; and trapping the target class of carbonnanotubes; and moving the target carbon nanotubes into the secondmicrofluidic layer.
 11. The method of claim 1, wherein the microfluidiclayer is water.
 12. An apparatus, comprising: a laser to emit a laserbeam having a frequency lower than a resonant frequency corresponding toa target class of carbon nanotubes, the resonant frequency determined bydiameter and chirality of the target class of carbon nanotubes; a firstmicrofluidic layer in laminar flow, the first microfluidic layer halvinga mixture of carbon nanotubes, the mixture of carbon nanotubes having atleast one target carbon nanotube; and a second microfluidic layer inlaminar flow, the second microfluidic layer in proximity with the firstfluid, the laser beam optically coupled to induce at least one opticaldipole trap in the target carbon nanotube and to move the target carbonnanotube into the second microfluidic layer.
 13. The apparatus of claim12, wherein the first and second microfluidic layers comprise water. 14.The apparatus of claim 12, wherein the target class of carbon nanotubesis metallic single-walled carbon nanotubes.
 15. The apparatus of claim12, wherein the target class of carbon nanotubes is semiconductorsingle-walled carbon nanotubes.
 16. The apparatus of claim 12, furthercomprising a third microfluidic layer in laminar flow, the thirdmicrofluidic layer in proximity with the second microfluidic layer. 17.The apparatus of claim 16, wherein the laser beam is coupled to emit anext frequency lower than a resonant frequency corresponding to a nexttarget class of carbon nanotubes in the mixture of carbon nanotubes, thenext resonant frequency determined by diameter and chirality of the nexttarget class of carbon nanotubes, the laser beam being optically coupledto trap the next target class of carbon nanotubes and to move the nexttarget class of carbon nanotubes into the third microfluidic layer. 18.The apparatus of claim 12, further comprising a first collector tocollect the target class of carbon nanotubes.
 19. A system, comprising:an apparatus coupled to direct a laser beam at a mixture of carbonnanotubes disposed in a microfluidic layer in laminar flow, the laserbeam having a laser frequency less than a resonant frequency of at leastone target class of carbon nanotubes, the resonant frequency determinedby diameter and chirality of the target class of carbon nanotubes, themixture including at least one target class of carbon nanotube, thelaser beam to move the target carbon nanotube into a second microfluidiclayer in laminar flow, the apparatus to collect the target carbonnanotube from the microfluidic layer in laminar flow; and apiezoelectric tube coupled to the collected target carbon nanotube. 20.The system of claim 19, further comprising a current amplifier coupledto the collected target carbon nanotube.
 21. The system of claim 20,further comprising a display coupled to the current amplifier.
 22. Anarticle of manufacture, comprising: a machine-accessible mediumincluding data that, when accessed by a machine, cause the machine toperform the operations comprising: directing a laser beam at a mixtureof carbon nanotubes disposed in a microfluidic layer in laminar flow,the laser beam having a frequency less than a resonant frequency of atleast one target class of carbon nanotubes, the resonant frequencydetermined by diameter and chirality of the target class of carbonnanotubes, the mixture including at least one target class of carbonnanotube; trapping at least one target carbon nanotube; and moving thetarget carbon nanotube into a second microfluidic layer in laminar flow.23. The article of manufacture of claim 22, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising identifying the resonant frequency ofthe target class of carbon nanotubes.
 24. The article of manufacture ofclaim 23, wherein the machine-accessible medium further includes datathat cause the machine to perform operations comprising identifying thediameter and chirality corresponding to the resonant frequency of thetarget class of carbon nanotubes.
 25. The article of manufacture ofclaim 24, wherein the machine-accessible medium further includes datathat cause the machine to perform operations comprising determining arelationship between diameter, chirality, and resonant frequency of thetarget class of carbon nanotubes.
 26. The article of manufacture ofclaim 22, wherein the machine-accessible medium further includes datathat cause the machine to perform operations comprising collecting thetarget carbon nanotube from the second microfluidic layer in laminarflow.
 27. The article of manufacture of claim 22, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising un-bundling the mixture of carbonnanotubes.